Hydraulic apparatus



Oct.29, 1968 V .R.SL|MM 3,407,744-

HYDRAUL I G APPARATUS Original Filed Aug. l2, 1965 2 Sheets-Sheet 1 9 R. 51.1 am

Oct. 29, 1968 v. R. SLIMM 3,407,744

HYDRAUL I C APPARATUS Original Filed Aug. 12, 1965 2 Sheets-Sheet 2 United States Patent 3,407,744 HYDRAULIC APPARATUS Victor R. Slimm, 1 Kenuif Road, Winchcombe, Cheltenham, Gloucester, England Continuation of application Ser. No. 479,231, Aug. 12, 1965. This application July 19, 1967, Ser. N0. 654,659

Claims priority, application Great Britain, Aug. 19, 1964,

2 Claims. (Cl. 103-162) ABSTRACT OF THE DISCLOSURE A hydraulic pump or motor is disclosed wherein the ported end portion of a rotary cylinder block having an assembly of cam operated pistons therein, is mounted to float in relation to a non-rotary valve member, and the lines of action of the parting and holding forces on the block are caused to remain substantially aligned with one another as the block rotates, so as to make it possible to use a holding force which is only slightly greater than the parting force, whereby the areas of the opposing surfaces of the valve member and the ported end portion of the block can be substantially reduced to minimize the energy loss which results from the shearing of liquid between the surfaces.

This is a continuation of application 479,231, filed Aug. 12, 1965, now abandoned.

This invention relates to hydraulic apparatus and more particularly to a hydraulic pump or to a hydraulic motor having the following features: a cylinder block rotatable about an axis passing through the block, cylinders in the cylinder block, the longitudinal axes of which are substantially parallel to the axis of rotation of the block, a cylinder port opening from each cylinder into a flat end surface of the cylinder block the centre of pressure of each cylinder port lying on or substantially on the longitudinal axis of the associated cylinder, pistons in the cylinders arranged to coact with means external to the block so as to be reciprocable in the cylinders by rotation of the block, a non-rotary valve having a fiat surface with a pair of main arcuate ports separated one from the other by two bridge portions of the flat surface, one port being arranged to have a supply function and the other port being arranged to have a return function, and each port in the flat end surface of the cylinder block co-acting, during rotation of the cylinder block, in turn with one main port, with a bridge portion, with the other main port and with the other bridge portion.

This pump or motor will be referred to as a pump or motor of the kind referred to. Normally a pump or motor of the kind referred to will be either a tilting head unit or a swash plate unit.

It is usual in a pump or motor of the kind referred to for the main ports to be of such arcuate length that, during block rotation, the number of cylinder ports in connection with each main port is not constant. Where the number N of cylinders in the block is an odd number, the number of cylinders in connection with each main port is alternately and N-l Where there are an even number N of cylinders in the block, there are alternately N 2 or ice cylinder ports alternately connecting with each main port. Where the greater number of cylinder ports connect with a main port, there is considerable overlap of the cylinder ports at the two ends of the main port so that the pressure area between the valve surface and the block surface is substantially larger than the main port. Where the smaller number of cylinder ports connect with a main port, there is little or no overlap at the ends of the main port, and the pressure area between the valve surface and the block surface is substantially the same as the main port. The centre of pressure of the hydraulic parting force developed between the two surfaces therefore varies its position during rotation of the cylinder block.

The pistons in connection with the cylinder ports respond to the pressure that exists in the cylinder ports and produce a holding force to maintain the block on the valve, the centre of pressure of this holding force varying in position depending on the number and angular position of cylinder ports connected to a main port at pressure. The centres of pressure of the holding and the parting forces normally will vary differently during block rotation producing a varying tipping moment on the block. It is customary to arrange that the holding force exceeds the parting force by about 15% in order to ensure that the block remains properly seated on the valve during the differing variations in position of the two centres of pressure. In turn this means that large surface areas must be provided on the faces of the cylinder block and valve to carry this excess load. These large surface areas are lubricated by a thin film of liquid between them but the continuous shearing of this liquid film during cylin der block rotation causes substantial power loss. The principal object of the present invention is to provide a structure of pump or motor of the kind referred to in which during block rotation the lines of action of the parting and the holding forces remain substantially aligned one with the other. Substantially aligned means that the lines of action are exactly aligned or that the lines of action are slightly non-aligned so as to produce a small tipping moment. This enables the holding force to be reduced and thus enables the surface areas of the valve and block surfaces to be reduced.

The present invention provides a pump or motor of the kind referred to in which at least one of the bridge portions has an arcuate length substantially greater than the arcuate length of each cylinder port, and including means affording floating movement of that part of the cylinder block having the Hat end surface, the arrangement being such that the resultant force on that part of the cylinder block having the flat .end surface which urges the flat end surface towards the flat surface of the valve is slightly greater than the resultant force generated hydraulically between the fiat end surface of the cylinder block and the fiat surface of the valve, and an auxiliary port in said one bridge portion restrictedly connected to one main port such that during rotation of the cylinder block each cylinder port coacts with the auxiliary port before passing over the said one main port, at least one dead centre position of each piston occurring as the associated cylinder port passes over a fixed position on said one bridge portion, the said fixed position being such that a cylinder port substantially at the said fixed position is just out of contact with one main port and such that during movement of the cylinder port over the bridge portion, piston movement relative to its dead centre position and connection of the cylinder port to the auxiliary port will adjust pressure of liquid in the cylinder smoothly towards the pressure of the one main port onto which its cylinder port will move from the bridge portion.

Each of the two bridge portions of the valve surface between the two adjacent pairs of main port ends may be arranged in the manner set out above.

The or each auxiliary port may be a groove extending from a main port into the associated bridge.

The term floating referred to the cylinder block means that the whole block or at lea t the part of the block having the block surface is capable of axial and tipping movement relative to the valve.

How the invention can be carried into effect will be particularly described with reference to the accompanying drawings in which:

FIGURE 1 is a cross section through the pump taken on the line I-'I of FIGURE 2 and FIGURE 2 is a plan view of thet valve surface of an axial piston pump.

Referring initially to FIGURE 1 a rotary cylinder block 1 is provided mounted for rotation on a valve 2 by means of a central shaft 3 secured to the valve 2 and extending into a bore 4 in the cylinder block. The shaft 3 locates the block loosely so permitting tipping and axial movement of the block relative to the valve i.e. the shaft permits floating movement of the block. The valve 2 has a valve surface 5 which appears in plan in FIGURE 1 whilst the cylinder block 1 has a co-operating annular surface 6 fitting closely to the valve surface 5. The annular surface 6 is the end surface of an annular projection at one end of the block. The surfaces 5 and 6 are both flat and are located at right angles to the axis of rotation of the cylinder block 1.

Within the cylinder block 1 a plurality of cylinders 7 are provided evenly spaced around the rotation axis, each having its axis parallel to the rotation axis. In each cylinder a piston 8 is provided for reciprocating movement therein. The pistons 8 extend from the open ends of the cylinder 7 remote from the valve surface 6. Each piston includes an internal bore 9 within which a connecting rod 11 is located. Each connecting 'rod engages its piston for transmission of thrust at a spherical surface 12. The connecting rod is retained within the piston by means of a circlip 13 engaging in cooperating grooves within the connecting rod and the bore 9. The connecting rod 11 extends from the piston and at its opposite end terminates in a ball 14. Each ball 14 is located within a pressure pad 15, all pads being fitted in regularly spaced arrangement around a drive flange 16 which faces the end of the cylinder block 1 remote from the valve surface 6. The drive flange 16 is carried by an integrally formed drive shaft 17 mounted for rotation in a bearing housing 18 having sleeve bearings 19 and 21. An axial thrust bearing for the drive flange 16 is formed by a wear plate 22 located within the bearing housing 18 and engaging the surface 23 of the drive flange 16. For lubrication of the surface 23 the drive flange bears against the wear plate 22 and passages 24 are provided extending one through each connecting rod to the surface 23. These passages will carry liquid at pressure from the cylinders.

The cylinder block 1 and valve plate 2 are mounted by means of a pair of arms 25 which engage by means of trunnions 26 in lugs 27 extending from the bearing housing 18. The arms 25 are secured to the valve plate 2 by means of bolts (not shown). The cylinder block 1, the valve plate 2, and the arm form 25 a tilting head tiltable relatively to the drive flange 16 about the axis of the trunnions 26. The axis of the trunnions 26 is arranged to pass through the drive flange 16 as a diameter of the circle passing through the centres of the ball joints 14.

As shown in FIGURE 1 the tilting head is so positioned about the trunnion axis that the axes of the drive flange 16 and the cylinder block 1 coincide. Rotation of the drive shaft 17 will cause rotation of the drive flange 16 and of the ball joints 14. Movement of the ball joints 14 will cause connecting rods 11 to engage the sides of the bores 9 in the pistons to cause the cylinder block 1 to rotate synchronously with the drive flange 16. With the tilting head in a position shown in FIGURE 1 rotation of the drive flange and cylinder block will not cause reciprocation of the pistons within their cylinders. By

moving the tilting head about the trunnion axis to cause inclination between the drive shaft axis and the cylinder block axis, it will be seen that rotation of the drive flange and cylinder block will then cause reciprocation of pistons Within their cylinders.

For the purpose of feeding liquid to and from the cylinders so that pump operation may be obtained, each cylinder 7 is provided with a port 28 opening into the surface 6 of the block. Each port 28 is of elongated shape and its centre is at a very slightly greater distance from the block axis than is the axis of the associated cylinder. Each port 28 co-operates during rotation of the cylinder block with fixed ports 29 and 31 which open into the valve surface 5. Hydraulic connections to the ports 29 and 31 may be effected either by flexible pipes or by means of pipes having swivelling couplings to accommodate movement of the tilting head.

For the purpose of ensuring that the valve surface remains in contact with block surface particularly during starting, a compression spring 41 is mounted on shaft 3 within a central recess 42 in the cylinder block. This spring 41 acts through a thrust bearing 43 on to a shoulder 44 of the block to urge the block towards the valve.

As will be seen from FIGURE 2 the main ports 29 and 31 are arcuate. The cylinder block 1 is mounted for rotation on top of the valve surface 5 as seen in the drawing in a clockwise sense. The main port 29 includes at its leading end having regard to the direction of rotation an auxiliary port 32 which comprises a groove of narrow width and comparatively shallow depth leading into the main port 29. The disposition of this groove 32 is such that during rotation of the cylinder block the cylinder ports pass over the groove 32 before passing over the main port 29. Similarly, the main port 31 includes an auxiliary port 33 similar in structure to the auxiliary port 32 and again located at the leading end so that in rotation of the cylinder block each cylinder port initially passes over the groove 33 before passing over the main port 31. In this embodiment the cylinder block includes seven cylinders from which seven cylinder ports 28 extend to the block surface 6 which rotates over the valve surface 5. The tilt axis of the trunnions 26 of the yoke are so arranged that the top and bottom dead centre positions of each piston occurs when the centre of the associated cylinder port lies at either of the positions A or B. The position A indicates the cylinder port centre where each piston attains its innermost position in its cylinder and the volume within the cylinder is at a minimum. The position B is the cylinder port centre where each piston is fully extended from its cylinder and each cylinder has its maximum volume. The positions B and A are positioned having regard to the main ports 29 and 31 such that as a cylinder port 28 is just leaving either of the main ports 29 and 31 and is just completely closed by the valve surface 5 when the associated piston is either at its top or at its bottom dead centre. The angular range of rotation between the two positions where a cylinder port 28 leaves one of the main ports and then makes connection with the other of the main ports is selected in accordance with the operating conditions to ensure a smooth pressure rise or fall in the associated cylinder. In the illustrated embodiment Where the block has seven cylinders, the operating speed is 2,000 rpm, the maximum pressure is about 5,000 p.s.i., and the maximum tilt angle of the tilting head is 25, the angle of rotation of each cylinder port when it leaves one main port until it connects with the other main port is then about 14.

The portions of the valve surface 5 between the adacent ends of the main ports are referred to as bridge portions of the valve surface.

In the illustrated embodiment the main port 29 is the high pressure port and the main port 31 is the low pressure port. A cylinder port 28 which just leaves the main port 29 will be at high pressure and immediately it leaves the port 29 its associated piston will be at its bottom dead centre position i.e. the cylinder will have minimum volume. Over about the next 6 of movement the associated piston will move away from its bottom dead centre to cause slight increase in cylinder volume, thus reducing the high pressure within the cylinder. Over the next 8 of movement the port 28 will make connection with the auxiliary port 33 which then allows restricted flow of liquid between the cylinder and the main port 31. The pressure in the cylinder port will now vary by virtue of two factors; one being the movement of the piston to increase cylinder volume and the other being the flow of liquid through the auxiliary port 33. By this means there is a substantially smooth pressure change-from the position at which the cylinder port 28 leaves the main port 29 to the position at which the cylinder port 28 fully connects with the main port 31. Most desirably, the pressure reduction as the cylinder port moves over this bridge portion is such that there is uniform loss of pressure per degree of angular movement of the block.

Where a cylinder port 28 passes from the main port 31 to the main port 29 it will initially be at the low pressure of the main port 31 and at the instant that it leaves the main port 31 its position will be at the top dead centre position at which the cylinder has maximum volume. Further movement over approximately 6 will cause movement of the piston to reduce the cylinder volume which will increase pressure within the cylinder and the cylinder port. Over the next 8 the cylinder port will connect with the auxiliary port 32 and How through this auxiliary port will assist in maintaining the smooth pressure change. During movement of the cylinder port 28 towards the main port 29 the rate of increase in pressure will depend on the rate of movement of the piston to reduce cylinder volume and the flow through the auxiliary port. If for example a large tilt angle is selected for the tilting head the rate of reduction of cylinder volume will be comparatively high and liquid will flow from the cylinder through the auxiliary port 32 into the main port 39 to prevent excessive pressure rise. Alternatively if the selected tilt angle for the tilting head is small, the pressure rise in the cylinder due to reduction in volume may not be sufficient and liquid will then flow from the main port 29 through the auxiliary port into the cylinder. Whichever direction liquid flows through the auxiliary port 32 the cylinder port 28 will be smoothly equalised to the pressure of the main port 29 as it moves over the auxiliary port 32 so that on making connection with the main port 29 it will be substantially at the pressure of the main port. In the case of the auxiliary port 33 liquid may also How in either direction when it connects to a cylinder port 28 passing from the main port 29 to the main port 31, the direction of flow depending on the selected tilt angle for the pump head, to ensure a smooth pressure change.

The smooth pressure rise and the smooth pressure fall at the two bridge portions will ensure principally that the centre of pressure of the hydraulic parting force acting between the valve surfaces 5 and 6 will vary in substantially the same manner as the centre of pressure of the holding force exerted hydraulically on the cylinder block by the pistons.

In the design of the pump illustrated in FIGURE 1 it is anranged that rotary drive is applied to the cylinder block 1 by virtue of engagement of the connecting rods 11 with the sides of the bores 9 in their pistons. Such rotary drive will impose a tipping moment on the cylinder block 1 which depends principally on the operating pressures in the ports 29 and 31. In the design of the cylinder block 1 and the valve 2 it is arranged that the centres of pressure of the parting force and the holding force may vary by a slight amount to produce a tilt on the cylinder block which opposes equally the tilting momentapplied by the connecting rods. In obtaining this result the cylinder port 28 for each cylinder is arranged so that its centre is a slightly greater distance away from the axis of the cylinder block rotation than is the longitudinal axis of the associated cylinder. In order to ensure that the holding force exceeds the parting force, by a very slight amount, some experimentation must be carried out to determine the inner and outer radii of the block surface 6, the inner and outer radii of the main ports 29 and 31 and the effective areas of the cylinder ports 28. The holding force which holds the cylinder block on to the valve surface 5 can be easily expressed as the pressures in the cylinders multiplied by the cross sectional areas of these cylinders. The parting force is far more difficult to determine since apart from liquid pressures acting between the surfaces 5 and 6 at the main ports, the cylinder ports and the auxiliary ports there is a leakage flow of pressure liquid between the surfaces 5 and 6 radially in either direction from the main ports and the pressure of such leakage of liquid will fall to a given low pressure when it reaches either the inner or the outer diameter of the cylinder block surface 6. The pressure of leakage liquid is additional to the pressure of liquid acting at the ports to determine the parting force which tends to urge the cylinder block away from the valve. Normally the 'area of each cylinder port 28 will be about three quarters of the cross sectional area of its associated cylinder. By the present invention the very small variation in tipping moment on the cylinder block enables the holding force to be arranged very slightly greater only than the parting force for successful operation. In fact the excess of the holding force over the parting force may be as little as 0.5 percent.

An additional advantage obtained by the invention is the comparative silence of operation when compared with a conventional pump or motor of the kind referred to. In such conventional pump or motor the passage of cylinder ports over the bridge portions involves very sudden pressure changes within the cylinders which cause considerable noise. The arrangement in accordance with the present invention by giving a smooth pressure rise and pressure fall in the cylinders as their cylinder ports pass over the bridge portions causes a considerable reduction in the operating noise level. A further advantage resulting from the invention is that there is very small leakage between the block and valve surfaces even at high pressure. As a result the radial width of the annular block surface need not be substantially greater than twice the radial width of the main ports.

In a pump or motor of the kind referred to arranged for variable displacement it is well known that the control forces which need to be exerted on the displacement varying member are of a rapidly varying and reversible nature, principally as the result of the sudden changes in pressure in cylinder ports moving over the bridge portions. In the present invention by arranging for smooth change of pressure in the cylinder ports passing over the bridge portions, the unexpected result is obtained that the control force which needs to be exerted on the displacement varying member is no longer reversible. It is then. possible to control the displacement by the use of a considerably smaller control force. In the illustrated embodiment the displacement varying member is the tilting head comprised by the valve 2, the cylinder block 1 and the arms 25. The tilting head is angularly adjustable about the trunnion pins 26 to vary displacement. The spring 41 is provided principally to provide initial engagement for the block and valve surfaces. During operation the spring may exert a force comparable with the excess of the holding force over the parting force.

The described embodiment of the invention shown in FIGURES 1 and 2 may be used as a variable displacement uni-directional motor if the valve 2 is slightly altered in position relative to the dead centre positions of the pistons. This alteration is such that the dead centre positions of the pistons now occur when the cylinder port centres are at the positions C and D shown in FIGURE 2. A cylinder port at position C or position D is just out of contact with the main port 31 or 29 on to which it will 7 move from the bridge. The geometry of the embodiment shown in FIGURE '1 in which the tilt axis of the trunnions 26 passes as a diameter through the circle containing the centres of ball joints 14 always ensures that the cylinder port centres corresponding to top and bottom dead centre piston positions as they appear on the valve surface will always lie substantially in a plane perpendicular to the tilt axis defined by the trunnions 26. Thus, when considering the illustrated embodiment for use as a motor the line II--II in FIGURE 2 indicates a line parallel to the tilt axis of the motor head. Assume for motor operation that the port 29 is the low pressure main port and the port 31 is the high pressure main port. A cylinder port 28 moving along the main port 29 will be at low pressure and as it passes from the end of the main port 29 and becomes closed by the valve surface 5 its centre will still be approaching the position C. Liquid contained in the cylinder will therefore be compressed by movement of the piston until the position C is attained. Each cylinder port 28 moves 14 from the position where it is first closed by the valve surface 5 on leaving the main port 29 before its associated pistons reaches its dead centre position at position C where it is just about to connect with port 31. During this movement the cylinder port 28 will connect with the auxiliary port 33 whose effect will combine with piston movement to adjust cylinder pressure smoothly to the high pressure of port 31. If the pressure in the main port 31 is not very high then the flow from the auxiliary port might be from the cylinder into the main port 31. Alternatively, if the pressure in the main port 31 is substantial the flow will be from the main port 31 into the cylinder. The operation however ensures that there is a reasonably smooth change in pressure from the position at which the port 28 leaves the main port 29 to the position at which the port 28 makes full connection with the main port 31.

Whilst the described embodiment shows the cylinder block floatable as a whole to cause surface 6 to seat on surface 5 it is equally within the scope of the invention to mount the cylinder block in bearings which permit only rotation relative to the valve and to fit a valve plate to the end of the block adjacent to the valve which rotates with the block and which engages by short sleeves into the ends of the cylinders through which cylinder ports extend to the plate surface engaging the valve. The block surface is then this surface of the plate which engages the surface of the valve. The short sleeves will permit floating movement of the plate onto the valve. The function and advantages are as for the described embodiment.

Whilst the described embodiment shows a tilting head unit the invention is equally applicable to a swash plate unit in which the cylinder block is capable of floating moment onto the valve.

Whilst the described embodiment shows a variable displacement pump it will be appreciated that the invention is equally applicable to a fixed displacement pump or motor of the kind referred to.

I claim as my invention: 1. A hydraulic pump or motor including: (a) a cylinder block rotatable in one direction about an axis passing through the block (b) said block having a plurality of cylinders, the longitudinal axes of which are substantially parallel to the axis of rotation of the block (c) said cylinders having ports opening into a flat end surface on one end portion of the cylinder block, the centers of pressure of which ports lie on or substantially on the longitudinal axes of the associated cylinders (d) pistons in the cylinders, and camming means external to the block arranged to reciprocate the pistons in the cylinders by rotation of the block (e) a valve member which is disposed on but fixed against rotation about the rotational axis of the block, and which has a flat surface with a pair of main arcuate valve ports therein separated one from the other by two bridge portions of the flat surface, one valve port being arranged to have a supply function and the other valve port being arranged to have a return function, and the valve member and cylinder block being cooperatively arranged so that each cylinder port in the fiat end surface of the cylinder block coacts during rotation in said direction of the cylinder block, in turn with one main port, with a bridge portion, with the other main port, and with the other bridge portion (f) means affording floating axial movement and fioating tilting movement of the aforesaid ported end portion of the cylinder block, the arrangement being such that the resultant force which urges the fiat end surface of the end portion towards the fiat surface of the valve member, is slightly greater than the resultant force generated hydrostatically between the flat surfaces of the cylinder block and the valve member (g) at least one bridge portion having an arcuate length substantially greater than the arcuate length of each cylinder port, and

(h) an auxiliary port in the aforesaid one bridge portion, restrictedly connected to the next main port in the said direction of rotation of the block, such that during rotation of the block in said direction each cylinder port coacts with the auxiliary port before passing over the aforesaid next main port (i) the camming means being arranged in relation to the main ports of the valve member so that during rotation of the cylinder block in the said direction, each piston achieves a dead center position at a fixed point on the one bridge portion at which the cylinder port corresponding thereto is just out of contact with one or the other of the main ports, and

(j) the pistons being reciprocated by the camming means through the medium of connecting rods, so that no appreciable side thrust is exerted on the cylinder block.

2. A hydraulic pump or motor including:

(a) a cylinder block rotatable in one direction about an axis passing through the block (b) said block having a plurality of cylinders, the longitudinal axes of which are substantially parallel to the axis of rotation of the block (c) said cylinders having ports opening into a flat end surface on one end portion of the cylinder block, the centers of pressure of which ports lie on or substantially on the longitudinal axes of the associated cylinders (d) pistons in the cylinders, and camming means external to the block arranged to reciprocate the pistons in the cylinders by rotation of the block (e) a valve member which is disposed on but fixed against rotation about the rotational axis of the block, and which has a fiat surface with a pair of main arcuate valve ports the-rein separated one from the other by two bridge portions of the Hat surface, one valve port being arranged to have a supply function and the other valve port being arranged to have a return function, and the valve member and cylinder block being cooperatively arranged so that each cylinder port in the flat end surface of the cylinder block coacts during rotation in said direction of the cylinder block, in turn with one main port, with a bridge portion, with the other main port, and with the other bridge portion (f) means affording floating axial movement and floating tilting movement of the cylinder block, the arrangement being such that the resultant force which urges the fiat end surface of the block therefor towards the flat surface of the valve member is slightly greater than the result-ant force generated hydrostatically between the flat surfaces of the cylinder block and the valve member (g) at least one bridge portion having an arcuate length substantially greater than the arcuate length of each cylinder port, and

(h) an auxiliary port in the aforesaid one bridge portion, restrictedly connected to the next main port in the said direction of rotation of the block, such that during rotation of the block in said direction each cylinder port coacts with the auxiliary port before passing over the aforesaid next main port (i) the c amming means being arranged in relation to the main ports of the valve member so that during the rotation of the cylinder block in the said direction, each piston achieves a dead center position at a fixed point on the one bridge portion at which the cylinder port corresponding thereto is just out of contact with one or the other of the main ports, and including (j) connecting rods secured articulatingly one to each piston (k) a bearing housing (1) a drive flange rotatably carried by the bearing housing and (m) an articulated connection for each connecting rod securing it to the drive flange, there being (n) a yoke supporting the valve member and the rotary cylinder block, and

(0) trunnions securing the yoke to the bearing housing to permit angular tilt movement of the yoke about the trunnion axis which is arranged to lie transversely to the rotation axis of the drive flange whereby the reciprocatory stroke of the pistons in their cylinders during rotation of the drive flange and the cylinder block may be varied in accordance with the tilt angle of the yoke (p) the connecting rods being arranged for limited articulation in their associated pistons whereby rotary motion of the drive flange is transmitted to the cylinder block, and

(q) the cylinder port for each cylinder being arranged so that its center is a slightly greater distance away from the axis of cylinder block rotation than is the longitudinal axis of the associated cylinder.

References Cited UNITED STATES PATENTS 2,161,143 6/1939 Doe et al. 103l62 2,298,850 10/1942 Vickers 103-162 2,619,041 11/1952 Born 103162 2,853,025 9/1958 Van Meter 103-162 3,037,489 6/1962 Douglas 103-162 X 3,200,761 8/1965 Firth et a1 103162 WILLIAM L. FREEH, Primary Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington,D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,407,744 October 29, 1968 Victor R. Slimm It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, lines 3 and 4, "Victor R. Slimm,- l Kenulf Road, Winchcombe, Cheltenham, Gloucester, England" should read Victor R. Slimm, Cheltenham, Gloucester, England, assignor to Dowty Technical Developments Limited, Brockhampton, Cheltenham, England, a company of Great Britain Signed and sealed this 10th day of'March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E.

Attesting Officer Commissioner of Patents 

